1. Field of the Invention
The present invention relates to a calibration method for radio frequency scattering parameter measurements; in particular, the present invention relates to a calibration method of one-tier radio frequency scattering parameter measurement de-embedding for semiconductor wafer devices or other substrate devices.
2. Description of Related Art
Typically, it is difficult to directly measure the voltage and current of a signal in the radio frequency microwave frequency band, thus in such a frequency band, it is necessary to discuss in the form of wave with actions through incidence, reflection and absorption thereby facilitating measurements of scattering parameters thereof. Because the entire measurement system needs to perform a sequence of complicated processes, the measurement calibration is consequently required in order to improve the accuracy of measurements, in which the measurement error can be mathematically characterized by using an error matrix, and the measurement error can be roughly divided into three major categories; i.e., random, drift and system errors, among which the scattering parameter of the system error can be effectively detected by a network analyzer under a stable measurement environment, further obtaining the error thereof, thus completing the measurement calibration.
In practice, the implementation procedure for such a calibration method is essentially to adjust the initial status of the instrument after startup to a user-defined actual measurement environment so as to eliminate any additional errors other than the tested object; while currently available radio frequency scattering parameter measurement for semiconductor wafer devices typically operates in a two-tier approach, comprising the following steps:                1. performing calibrations on the system before starting the measurement thereby eliminating the effect caused by the measuring instrument and environment; hence it first uses a probe in conjunction with an Impedance Standard Substrate (ISS) for calibration, whose calibration method can be SOLT (Short-Open-Load-Thru) or LRM (Line-Reflect-Match), and then moves the measurement reference plane to the tip of the probe, but a small segment of connection line exists between the probe pad and the tested device within the wafer, and the capacitive effect in the probe pad of large area may not be effectively calibrated;        2. further performing calibrations on the additional dummy structure (e.g., Short, Open, Thru etc.) of the wafer so as to remove the effects caused by the pad and the connection line, i.e., the de-embedding procedure, thus the major purpose of de-embedding is to remove the effect of the test fixture from the measurement data in raw test results so as to acquire the most primitive characterization of the device.        
However, such a two-tier measurement approach has the following drawbacks:                1. the high frequency feature of the additional dummy structure on the wafer may not be conveniently appreciated, and in case it is assumed to be an ideal feature, significant errors may be undesirably introduced at high frequency in the de-embedding process;        2. the two-tier measurement consumes much the wafer probe test time, consequently, as applying to massive tests, it becomes comparatively critical;        3. since the Impedance Standard Substrate (ISS) is expensive and the feature thereof may degrade after each test due to scratches on its pad caused by the probe, the substrate needs to be replaced after a certain cycles of use, thus adversely elevating the test cost.        
Regarding to the aforementioned drawbacks, a few literatures have proposed certain solutions therefore, including:
1. IEEE Trans. Electron Devices, vol. 54, no. 10, pp. 2706-2714, October 2007, describing the use of a one-tier measurement for de-embedding operation at the cost of five dummy structures (Open, Short, Thru, Left, Right), so the precision thereof may be compromised in comparison with the two-tier approach.
2. IEEE Trans. Microwave Theory Tech., vol. 51, pp. 2391-2401, December 2003, describing a Multiline Thru-Reflect-Line (TRL) calibration method developed by NIST (National Institute of Standards and Technology), which enables completion of calibration and de-embedding process in a one-tier fashion, but presents a disadvantage of requirement on multiple transmission line segments which significantly occupies valuable wafer area.
As a result, it would be an optimal solution to provide a calibration method for radio frequency scattering parameter measurements which can perform the de-embedding process in a one-tier radio frequency scattering parameter measurement of semiconductor wafer devices or other substrate devices without using the Impedance Standard Substrate but requiring the resolution of simply three calibrators for operations.